Rotating polygon scanning mirrors are typically used in laser printers to provide a “raster” scan of the image of a laser light source across a moving photosensitive medium, such as a rotating drum. Such a system requires that the rotation of the photosensitive drum and the rotating polygon mirror be synchronized so that the beam of light (laser beam) sweeps or scans across the rotating drum in one direction as a facet of the polygon mirror rotates past the laser beam. The next facet of the rotating polygon mirror generates a similar scan or sweep which also traverses the rotating photosensitive drum but provides an image line that is spaced or displaced from the previous image line.
The rotational speed of a typical polygon mirror can be varied over a small range, but significantly higher rotational speeds requires more advanced and robust bearing technology which, of course, means significantly higher manufacturing costs. Because the cost of a polygon mirror increases significantly as the printer speed increases, it is not economical to use mirrors suitable for high speed printing with slower fixed speed printers. Also, multi-speed printers that provide both high speed and slow speed printing typically require a different polygon mirror for each of the different speeds. Consequently, printer manufacturers typically must maintain a large inventory of different polygon mirrors to cover the range of printer speeds offered for sale.
There have also been prior art efforts to use a less expensive flat mirror with a single reflective surface, such as a resonant mirror, to provide a scanning beam. For example, a single axis scanning mirror may be used to generate the beam sweep or scan instead of a rotating polygon mirror. The rotating photosensitive drum and the scanning mirror are synchronized as the “resonant” mirror first pivots or rotates in one direction to produce a printed image line on the medium that is at right angles or orthogonal with the movement of the photosensitive medium. However, the return sweep will traverse a trajectory on the moving photosensitive drum that is at an angle with the printed image line resulting from the previous sweep. Consequently, use of a single reflecting surface resonant mirror according to the prior art required that the modulation of the reflected light beam be interrupted as the mirror completed the return sweep or cycle, and then again start scanning in the original direction. Using only one of the sweep directions of the mirror, of course, reduces the print speed and requires expensive and sophisticated synchronization of stops and starts of the rotating drum. Therefore, to effectively use an inexpensive resonant mirror requires that the mirror surface be continuously and easily adjusted in a direction perpendicular to the scan such that the resonant sweep of the mirror in each direction generates images on a moving or rotating photosensitive drum that are always parallel. This continuous perpendicular movement may be accomplished by the use of a dual axis torsional mirror, or a pair of single axis mirrors. Of course, either of these solutions is more expensive than using one single frequency scanning mirror.
Texas Instruments presently manufactures torsional axis analog mirror MEMS devices fabricated out of a single piece of material (such as silicon, for example) typically having a thickness of about 100-115 microns. A dual axis version layout consists of a mirror supported on a gimbal frame by two silicon torsional hinges. The mirror may be of any desired shape, although an oval shape is typically preferred. An elongated oval shaped mirror having a long axis of about 4.0 millimeters and a short axis of about 1.5 millimeters has been found to be especially suitable. The gimbal frame is attached to a support frame by another set of torsional hinges. This dual axis Texas Instruments' manufactured mirror has been found to be particularly suitable for use with a laser printer. A similar Texas Instruments' single axis mirror device is also fabricated by simply eliminating the gimbal frame and hinging the mirror directly to the support structure. One example of a dual axis torsional hinged mirror is disclosed in U.S. Pat. No. 6,295,154 entitled “Optical Switching Apparatus” and was assigned to the same assignee on the present invention.
Although MEMS type torsional hinged scanning mirrors are less expensive than polygon mirrors, they are designed to have a single resonant frequency within a rather narrow frequency band. Consequently, an inventory of different mirrors for different print speeds is still considered necessary.
Therefore, it will be appreciated that if a single resonant frequency scanning mirror could be used for both multi-speed printers and a series of printers having different fixed print speeds, manufacturing costs and inventory costs could be significantly reduced.